This invention relates to a block deformation removing filter, an image processing apparatus using the filter and a method of filtering image signals. Particularly, the present invention relates to a block deformation (or blocking noise) removing filter which is used to remove a block deformation (or blocking noise) taking place when for example an image is encoded by compression and the encoded image signal is decoded, and an image processing apparatus using the block deformation removing filter.
When an image is encoded by compression with high efficiency by way of an image compression coding technique, it may often present an image degradation associated with various compression coding techniques. For example, when an image is encoded by compression in units of block by way of the discrete cosine transformation (DCT), the block boundaries may become discontinuous to each another, thus resulting in block deformation.
The possible block deformation is most outstanding image quality degradation in a visual sense when a compression coding technique is applied in units of block, so that the improvement of the image quality requires minimization or removal of such deformation or discontinuity. So far in order to solve this problem, a block deformation removing filter is used to remove block deformation by applying filtering processing on block boundaries.
FIGS. 1A and 1B are illustrations showing how to remove the block deformation by way of a low-pass filter. FIG. 1A shows intensity values of decoded image before filtering, while FIG. 1B after filtering. In this connection, FIGS. 1A and 1B respectively show four scanning lines as representation which cross a block boundary in the image at right angle, and the heights of respective lines show intensity levels or values (shown by solid circles) of pixels lined up on each line.
FIG. 1A shows differentials A in intensity level taking place on every line crossing at right angle with the block boundary shown by a dotted line, where the differentials may be slightly different but essentially same, which cause a discontinuous block boundary resulting in a block deformation. Such a block deformation contributes to assume a mosaic pattern of a decoded image in those points, thus causing a significant visual difficulty.
Therefore, as shown in FIG. 1B, possible differentials xcex94 in the intensity level at the block boundary are smoothed by way of low-pass filter, thereby taking the straight edge off in intensity value, resulting in a minimized block deformation.
Block deformation may occur not only on block boundaries but also on other positions, for example, when motion compensation is carried out in moving picture coding.
However, a conventional block deformation removing filter can remove block deformation only on the block boundaries. And, hence, no complete removal of block deformation is achieved.
A purpose. of the present invention is to provide a block deformation removing filter, an image signal processing apparatus with the block deformation removing filter, a method of filtering an image signal, and a storage medium for storing software therefor, to remove block deformation that occurs not only a block boundary but also outside the boundary of an image signal.
The present invention provides a block deformation removing filter comprising: a detector responsive to pixel signals included in a plurality of pixel blocks forming an image to detect a plurality of differentials each between at least two pixel signal levels on block boundaries between the pixel blocks and in the vicinity of the block boundaries; a comparator to compare the differentials with each other to obtain the maximum differential absolute value; a determiner to compare the maximum differential absolute value with a reference value to determine whether block deformation occurs in a pixel signal in the vicinity of a position on the pixel blocks where the maximum differential absolute value is obtained; and a processor to remove the block deformation when it is determined that the block deformation occurs in the pixel signal in the vicinity of the position on the pixel blocks where the maximum differential absolute value is obtained.
Furthermore, the present invention provides an image processing apparatus comprising: a decoder to decode pixel signals included in a plurality of pixel blocks forming an encoded image; a detector responsive to the decoded pixel signals to detect a plurality of differentials each between at least two pixel signal levels on block boundaries between the pixel blocks and in the vicinity of the block boundaries; a comparator to compare the differentials with each other to obtain the maximum differential absolute value; a determiner to compare the maximum differential absolute value with a reference value to determine whether block deformation occurs in a pixel signal in the vicinity of a position on the pixel blocks where the maximum differential absolute value is obtained; and a processor to remove the block deformation when it is determined that the block deformation occurs in the pixel signal in the vicinity of the position on the pixel blocks where the maximum differential absolute value is obtained.
Furthermore, the present invention provides an image processing apparatus comprising: an encoder to encode predictive error signals for pixel signals included in a plurality of pixel blocks forming an encoded image; a decoder to decode the encoded predictive error signals; a detector responsive to the decoded predictive error signals to detect a plurality of differentials each between at least two pixel signal levels on block boundaries between the pixel blocks and in the vicinity of the block boundaries; a comparator to compare the differentials with each other to obtain the maximum differential absolute value; a determiner to compare the maximum differential absolute value with a reference value to determine whether block deformation occurs in a pixel signal in the vicinity of a position on the pixel blocks where the maximum differential absolute value is obtained; a processor to remove the block deformation when it is determined that the block deformation occurs in the pixel signal in the vicinity of the position on the pixel blocks where the maximum differential absolute value is obtained; a predictor to generate predictive signals based on output signals of the processor; and a generator to generate the predictive error signals by subtracting the predictive signals from the pixel signals.
Furthermore, the present invention provides an image processing apparatus comprising: a decoder to decode pixel signals included in a plurality of pixel blocks forming an encoded image; an adder to add the decoded pixel signals and predictive signals; a detector responsive to output signals of the adder to detect a plurality of differentials each between at least two pixel signal levels on block boundaries between the pixel blocks and in the vicinity of the block boundaries; a comparator to compare the differentials with each other to obtain the maximum differential absolute value; a determiner to compare the maximum differential absolute value with a reference value to determine whether block deformation occurs in a pixel signal in the vicinity of a position on the pixel blocks where the maximum differential absolute value is obtained; a processor to remove the block deformation when it is determined that the block deformation occurs in the pixel signal in the vicinity of the position on the pixel blocks where the maximum differential absolute value is obtained; and a predictor to generate the predictive signals based on output signals of the processor.
Furthermore, the present invention provides a method of filtering image signals comprising the steps of: detecting, in response to pixel signals included in a plurality of pixel blocks forming an image, a plurality of differentials each between at least two pixel signal levels on block boundaries between the pixel blocks and in the vicinity of the block boundaries; comparing the differentials with each other to obtain the maximum differential absolute value; comparing the maximum differential absolute value with a reference value to determine whether block deformation occurs in a pixel signal in the vicinity of a position on the pixel blocks where the maximum differential absolute value is obtained; and removing the block deformation when it is determined that the block deformation occurs in the pixel signal in the vicinity of the position on the pixel blocks where the maximum differential absolute value is obtained.
Furthermore, the present invention provides a computer readable storage medium storing a program for causing a computer to filter image signals comprising: a program code, responsive to pixel signals included in a plurality of pixel blocks forming an image, to detect a plurality of differentials each between at least two pixel signal levels on block boundaries between the pixel blocks and in the vicinity of the block boundaries; a program code to compare the differentials with each other to obtain the maximum differential absolute value; a program code to compare the maximum differential absolute value with a reference value to determine whether block deformation occurs in a pixel signal in the vicinity of a position on the pixel blocks where the maximum differential absolute value is obtained; and a program code to remove the block deformation when it is determined that the block deformation occurs in the pixel signal in the vicinity of the position on the pixel blocks where the maximum differential absolute value is obtained.